Age-related macular degeneration (AMD) is the leading cause of visual loss among older adults in developed countries. Complement factor H (CFH) and CFH-related proteins (CFHR1 to CFHR5) are implicated in the development of AMD. These related proteins are encoded in the Regulation of Complement Activation (RCA) gene cluster in human chromosome 1q31. Two major protein variants of CFH, Y402H and I62V, are strongly associated with risk of AMD. A homozygous deletion in CFHR3/CFHR1 is protective in AMD. The Y402H and I62V risk variants have structural changes that impair anti-inflammatory and regulatory CFH functions. CFHR proteins compete with CFH at several binding sites. Although complement proteins encoded in the RCA gene cluster are strongly implicated by genetic studies in the pathogenesis of AMD, a major gap in knowledge is how systemic levels of CFH and the five CFHR proteins are related to AMD. This multi-center, collaborative study will use an epidemiological approach to advance knowledge from the genetic level to the expression of circulating proteins and their variants and isoforms. We have developed a novel multiplexed multiple reaction monitoring (MRM) assay based upon mass spectrometry that can measure the plasma levels of all four variants of CFH. MRM overcomes the limitations of immunoassays in detection of highly similar homologues and sequence variants such as that found in CFH and CFHR proteins. Our pilot studies suggest that systemic CFH risk variants are associated with AMD. We propose, under the support of this proposal, to further develop the MRM assay to include all five CFHR proteins and their isoforms. We hypothesize: (1) risk of AMD is associated with plasma concentrations of CFH variants Y402H and I62V and plasma concentrations of complement factor H-related proteins CFHR1 to CFHR5, (2) common single nucleotide polymorphisms (SNPs) and other factors are associated with variations in plasma levels of both CFH variants and CFHR proteins. The specific aims are to: (1) characterize the relationship between plasma CFH Y402H and I62V variant levels and risk of AMD, (2) develop MRM assays for measuring plasma CFHR1 to CFHR5, (3) characterize the relationship between plasma CFHR1 to CFHR5 levels and risk of AMD, and (4) identify SNPs and other factors associated with plasma levels of CFH variants and CFHR1 to CFHR5 through a genome-wide association study (GWAS). To address these aims, we will examine the relationship of plasma CFH Y402, H402, I62, and V62 variants and CFHR1 to CFHR5 at baseline in 4,907 participants in the Age, Gene/Environment Susceptibility-Reykjavik (AGES-Reykjavik) Study with prevalent and incident AMD. A GWAS will identify SNPs associated with respective CFH and CFHR protein levels. The AGES-Reykjavik Study is a population-based epidemiological study aimed at identifying factors that contribute to disease in older adults. This study will evaluate promising candidate biomarkers and help to determine whether systemic CFH and CFHR proteins play a role in AMD and represent a potential pathway for risk stratification and/or therapeutic intervention.